crossfilter/src/crossfilter.js

Fast multidimensional filtering for coordinated views.

exports.crossfilter = crossfilter;

function crossfilter() {
  var crossfilter = {
    add: add,
    remove: removeData,
    dimension: dimension,
    groupAll: groupAll,
    size: size
  };

  var data = [], // the records
      n = 0, // the number of records; data.length
      m = 0, // a bit mask representing which dimensions are in use
      M = 8, // number of dimensions that can fit in `filters`
      filters = crossfilter_array8(0), // M bits per record; 1 is filtered out
      filterListeners = [], // when the filters change
      dataListeners = [], // when data is added
      removeDataListeners = []; // when data is removed

  // Adds the specified new records to this crossfilter.
  function add(newData) {
    var n0 = n,
        n1 = newData.length;

    // If there's actually new data to add…
    // Merge the new data into the existing data.
    // Lengthen the filter bitset to handle the new records.
    // Notify listeners (dimensions and groups) that new data is available.
    if (n1) {
      data = data.concat(newData);
      filters = crossfilter_arrayLengthen(filters, n += n1);
      dataListeners.forEach(function(l) { l(newData, n0, n1); });
    }

    return crossfilter;
  }

  // Removes all records that match the current filters.
  function removeData() {
    var newIndex = crossfilter_index(n, n),
        removed = [];
    for (var i = 0, j = 0; i < n; ++i) {
      if (filters[i]) newIndex[i] = j++;
      else removed.push(i);
    }

    // Remove all matching records from groups.
    filterListeners.forEach(function(l) { l(0, [], removed); });

    // Update indexes.
    removeDataListeners.forEach(function(l) { l(newIndex); });

    // Remove old filters and data by overwriting.
    for (var i = 0, j = 0, k; i < n; ++i) {
      if (k = filters[i]) {
        if (i !== j) filters[j] = k, data[j] = data[i];
        ++j;
      }
    }
    data.length = j;
    while (n > j) filters[--n] = 0;
  }

  // Adds a new dimension with the specified value accessor function.
  function dimension(value) {
    var dimension = {
      filter: filter,
      filterExact: filterExact,
      filterRange: filterRange,
      filterFunction: filterFunction,
      filterAll: filterAll,
      top: top,
      bottom: bottom,
      group: group,
      groupAll: groupAll,
      dispose: dispose,
      remove: dispose // for backwards-compatibility
    };

    var one = ~m & -~m, // lowest unset bit as mask, e.g., 00001000
        zero = ~one, // inverted one, e.g., 11110111
        values, // sorted, cached array
        index, // value rank ↦ object id
        newValues, // temporary array storing newly-added values
        newIndex, // temporary array storing newly-added index
        sort = quicksort_by(function(i) { return newValues[i]; }),
        refilter = crossfilter_filterAll, // for recomputing filter
        refilterFunction, // the custom filter function in use
        indexListeners = [], // when data is added
        dimensionGroups = [],
        lo0 = 0,
        hi0 = 0;

    // Updating a dimension is a two-stage process. First, we must update the
    // associated filters for the newly-added records. Once all dimensions have
    // updated their filters, the groups are notified to update.
    dataListeners.unshift(preAdd);
    dataListeners.push(postAdd);

    removeDataListeners.push(removeData);

    // Incorporate any existing data into this dimension, and make sure that the
    // filter bitset is wide enough to handle the new dimension.
    m |= one;
    if (M >= 32 ? !one : m & -(1 << M)) {
      filters = crossfilter_arrayWiden(filters, M <<= 1);
    }
    preAdd(data, 0, n);
    postAdd(data, 0, n);

    // Incorporates the specified new records into this dimension.
    // This function is responsible for updating filters, values, and index.
    function preAdd(newData, n0, n1) {

      // Permute new values into natural order using a sorted index.
      newValues = newData.map(value);
      newIndex = sort(crossfilter_range(n1), 0, n1);
      newValues = permute(newValues, newIndex);

      // Bisect newValues to determine which new records are selected.
      var bounds = refilter(newValues), lo1 = bounds[0], hi1 = bounds[1], i;
      if (refilterFunction) {
        for (i = 0; i < n1; ++i) {
          if (!refilterFunction(newValues[i], i)) filters[newIndex[i] + n0] |= one;
        }
      } else {
        for (i = 0; i < lo1; ++i) filters[newIndex[i] + n0] |= one;
        for (i = hi1; i < n1; ++i) filters[newIndex[i] + n0] |= one;
      }

      // If this dimension previously had no data, then we don't need to do the
      // more expensive merge operation; use the new values and index as-is.
      if (!n0) {
        values = newValues;
        index = newIndex;
        lo0 = lo1;
        hi0 = hi1;
        return;
      }

      var oldValues = values,
          oldIndex = index,
          i0 = 0,
          i1 = 0;

      // Otherwise, create new arrays into which to merge new and old.
      values = new Array(n);
      index = crossfilter_index(n, n);

      // Merge the old and new sorted values, and old and new index.
      for (i = 0; i0 < n0 && i1 < n1; ++i) {
        if (oldValues[i0] < newValues[i1]) {
          values[i] = oldValues[i0];
          index[i] = oldIndex[i0++];
        } else {
          values[i] = newValues[i1];
          index[i] = newIndex[i1++] + n0;
        }
      }

      // Add any remaining old values.
      for (; i0 < n0; ++i0, ++i) {
        values[i] = oldValues[i0];
        index[i] = oldIndex[i0];
      }

      // Add any remaining new values.
      for (; i1 < n1; ++i1, ++i) {
        values[i] = newValues[i1];
        index[i] = newIndex[i1] + n0;
      }

      // Bisect again to recompute lo0 and hi0.
      bounds = refilter(values), lo0 = bounds[0], hi0 = bounds[1];
    }

    // When all filters have updated, notify index listeners of the new values.
    function postAdd(newData, n0, n1) {
      indexListeners.forEach(function(l) { l(newValues, newIndex, n0, n1); });
      newValues = newIndex = null;
    }

    function removeData(reIndex) {
      for (var i = 0, j = 0, k; i < n; ++i) {
        if (filters[k = index[i]]) {
          if (i !== j) values[j] = values[i];
          index[j] = reIndex[k];
          ++j;
        }
      }
      values.length = j;
      while (j < n) index[j++] = 0;

      // Bisect again to recompute lo0 and hi0.
      var bounds = refilter(values);
      lo0 = bounds[0], hi0 = bounds[1];
    }

    // Updates the selected values based on the specified bounds [lo, hi].
    // This implementation is used by all the public filter methods.
    function filterIndexBounds(bounds) {
      var lo1 = bounds[0],
          hi1 = bounds[1];

      if (refilterFunction) {
        refilterFunction = null;
        filterIndexFunction(function(d, i) { return lo1 <= i && i < hi1; });
        lo0 = lo1;
        hi0 = hi1;
        return dimension;
      }

      var i,
          j,
          k,
          added = [],
          removed = [];

      // Fast incremental update based on previous lo index.
      if (lo1 < lo0) {
        for (i = lo1, j = Math.min(lo0, hi1); i < j; ++i) {
          filters[k = index[i]] ^= one;
          added.push(k);
        }
      } else if (lo1 > lo0) {
        for (i = lo0, j = Math.min(lo1, hi0); i < j; ++i) {
          filters[k = index[i]] ^= one;
          removed.push(k);
        }
      }

      // Fast incremental update based on previous hi index.
      if (hi1 > hi0) {
        for (i = Math.max(lo1, hi0), j = hi1; i < j; ++i) {
          filters[k = index[i]] ^= one;
          added.push(k);
        }
      } else if (hi1 < hi0) {
        for (i = Math.max(lo0, hi1), j = hi0; i < j; ++i) {
          filters[k = index[i]] ^= one;
          removed.push(k);
        }
      }

      lo0 = lo1;
      hi0 = hi1;
      filterListeners.forEach(function(l) { l(one, added, removed); });
      return dimension;
    }

    // Filters this dimension using the specified range, value, or null.
    // If the range is null, this is equivalent to filterAll.
    // If the range is an array, this is equivalent to filterRange.
    // Otherwise, this is equivalent to filterExact.
    function filter(range) {
      return range == null
          ? filterAll() : Array.isArray(range)
          ? filterRange(range) : typeof range === "function"
          ? filterFunction(range)
          : filterExact(range);
    }

    // Filters this dimension to select the exact value.
    function filterExact(value) {
      return filterIndexBounds((refilter = crossfilter_filterExact(bisect, value))(values));
    }

    // Filters this dimension to select the specified range [lo, hi].
    // The lower bound is inclusive, and the upper bound is exclusive.
    function filterRange(range) {
      return filterIndexBounds((refilter = crossfilter_filterRange(bisect, range))(values));
    }

    // Clears any filters on this dimension.
    function filterAll() {
      return filterIndexBounds((refilter = crossfilter_filterAll)(values));
    }

    // Filters this dimension using an arbitrary function.
    function filterFunction(f) {
      refilter = crossfilter_filterAll;

      filterIndexFunction(refilterFunction = f);

      lo0 = 0;
      hi0 = n;

      return dimension;
    }

    function filterIndexFunction(f) {
      var i,
          k,
          x,
          added = [],
          removed = [];

      for (i = 0; i < n; ++i) {
        if (!(filters[k = index[i]] & one) ^ !!(x = f(values[i], i))) {
          if (x) filters[k] &= zero, added.push(k);
          else filters[k] |= one, removed.push(k);
        }
      }
      filterListeners.forEach(function(l) { l(one, added, removed); });
    }

    // Returns the top K selected records based on this dimension's order.
    // Note: observes this dimension's filter, unlike group and groupAll.
    function top(k) {
      var array = [],
          i = hi0,
          j;

      while (--i >= lo0 && k > 0) {
        if (!filters[j = index[i]]) {
          array.push(data[j]);
          --k;
        }
      }

      return array;
    }

    // Returns the bottom K selected records based on this dimension's order.
    // Note: observes this dimension's filter, unlike group and groupAll.
    function bottom(k) {
      var array = [],
          i = lo0,
          j;

      while (i < hi0 && k > 0) {
        if (!filters[j = index[i]]) {
          array.push(data[j]);
          --k;
        }
        i++;
      }

      return array;
    }

    // Adds a new group to this dimension, using the specified key function.
    function group(key) {
      var group = {
        top: top,
        all: all,
        reduce: reduce,
        reduceCount: reduceCount,
        reduceSum: reduceSum,
        order: order,
        orderNatural: orderNatural,
        size: size,
        dispose: dispose,
        remove: dispose // for backwards-compatibility
      };

      // Ensure that this group will be removed when the dimension is removed.
      dimensionGroups.push(group);

      var groups, // array of {key, value}
          groupIndex, // object id ↦ group id
          groupWidth = 8,
          groupCapacity = crossfilter_capacity(groupWidth),
          k = 0, // cardinality
          select,
          heap,
          reduceAdd,
          reduceRemove,
          reduceInitial,
          update = crossfilter_null,
          reset = crossfilter_null,
          resetNeeded = true,
          groupAll = key === crossfilter_null;

      if (arguments.length < 1) key = crossfilter_identity;

      // The group listens to the crossfilter for when any dimension changes, so
      // that it can update the associated reduce values. It must also listen to
      // the parent dimension for when data is added, and compute new keys.
      filterListeners.push(update);
      indexListeners.push(add);
      removeDataListeners.push(removeData);

      // Incorporate any existing data into the grouping.
      add(values, index, 0, n);

      // Incorporates the specified new values into this group.
      // This function is responsible for updating groups and groupIndex.
      function add(newValues, newIndex, n0, n1) {
        var oldGroups = groups,
            reIndex = crossfilter_index(k, groupCapacity),
            add = reduceAdd,
            initial = reduceInitial,
            k0 = k, // old cardinality
            i0 = 0, // index of old group
            i1 = 0, // index of new record
            j, // object id
            g0, // old group
            x0, // old key
            x1, // new key
            g, // group to add
            x; // key of group to add

        // If a reset is needed, we don't need to update the reduce values.
        if (resetNeeded) add = initial = crossfilter_null;

        // Reset the new groups (k is a lower bound).
        // Also, make sure that groupIndex exists and is long enough.
        groups = new Array(k), k = 0;
        groupIndex = k0 > 1 ? crossfilter_arrayLengthen(groupIndex, n) : crossfilter_index(n, groupCapacity);

        // Get the first old key (x0 of g0), if it exists.
        if (k0) x0 = (g0 = oldGroups[0]).key;

        // Find the first new key (x1), skipping NaN keys.
        while (i1 < n1 && !((x1 = key(newValues[i1])) >= x1)) ++i1;

        // While new keys remain…
        while (i1 < n1) {

          // Determine the lesser of the two current keys; new and old.
          // If there are no old keys remaining, then always add the new key.
          if (g0 && x0 <= x1) {
            g = g0, x = x0;

            // Record the new index of the old group.
            reIndex[i0] = k;

            // Retrieve the next old key.
            if (g0 = oldGroups[++i0]) x0 = g0.key;
          } else {
            g = {key: x1, value: initial()}, x = x1;
          }

          // Add the lesser group.
          groups[k] = g;

          // Add any selected records belonging to the added group, while
          // advancing the new key and populating the associated group index.
          while (!(x1 > x)) {
            groupIndex[j = newIndex[i1] + n0] = k;
            if (!(filters[j] & zero)) g.value = add(g.value, data[j]);
            if (++i1 >= n1) break;
            x1 = key(newValues[i1]);
          }

          groupIncrement();
        }

        // Add any remaining old groups that were greater than all new keys.
        // No incremental reduce is needed; these groups have no new records.
        // Also record the new index of the old group.
        while (i0 < k0) {
          groups[reIndex[i0] = k] = oldGroups[i0++];
          groupIncrement();
        }

        // If we added any new groups before any old groups,
        // update the group index of all the old records.
        if (k > i0) for (i0 = 0; i0 < n0; ++i0) {
          groupIndex[i0] = reIndex[groupIndex[i0]];
        }

        // Modify the update and reset behavior based on the cardinality.
        // If the cardinality is less than or equal to one, then the groupIndex
        // is not needed. If the cardinality is zero, then there are no records
        // and therefore no groups to update or reset. Note that we also must
        // change the registered listener to point to the new method.
        j = filterListeners.indexOf(update);
        if (k > 1) {
          update = updateMany;
          reset = resetMany;
        } else {
          if (!k && groupAll) {
            k = 1;
            groups = [{key: null, value: initial()}];
          }
          if (k === 1) {
            update = updateOne;
            reset = resetOne;
          } else {
            update = crossfilter_null;
            reset = crossfilter_null;
          }
          groupIndex = null;
        }
        filterListeners[j] = update;

        // Count the number of added groups,
        // and widen the group index as needed.
        function groupIncrement() {
          if (++k === groupCapacity) {
            reIndex = crossfilter_arrayWiden(reIndex, groupWidth <<= 1);
            groupIndex = crossfilter_arrayWiden(groupIndex, groupWidth);
            groupCapacity = crossfilter_capacity(groupWidth);
          }
        }
      }

      function removeData() {
        if (k > 1) {
          var oldK = k,
              oldGroups = groups,
              seenGroups = crossfilter_index(oldK, oldK);

          // Filter out non-matches by copying matching group index entries to
          // the beginning of the array.
          for (var i = 0, j = 0; i < n; ++i) {
            if (filters[i]) {
              seenGroups[groupIndex[j] = groupIndex[i]] = 1;
              ++j;
            }
          }

          // Reassemble groups including only those groups that were referred
          // to by matching group index entries.  Note the new group index in
          // seenGroups.
          groups = [], k = 0;
          for (i = 0; i < oldK; ++i) {
            if (seenGroups[i]) {
              seenGroups[i] = k++;
              groups.push(oldGroups[i]);
            }
          }

          if (k > 1) {
            // Reindex the group index using seenGroups to find the new index.
            for (var i = 0; i < j; ++i) groupIndex[i] = seenGroups[groupIndex[i]];
          } else {
            groupIndex = null;
          }
          filterListeners[filterListeners.indexOf(update)] = k > 1
              ? (reset = resetMany, update = updateMany)
              : k === 1 ? (reset = resetOne, update = updateOne)
              : reset = update = crossfilter_null;
        } else if (k === 1) {
          if (groupAll) return;
          for (var i = 0; i < n; ++i) if (filters[i]) return;
          groups = [], k = 0;
          filterListeners[filterListeners.indexOf(update)] =
          update = reset = crossfilter_null;
        }
      }

      // Reduces the specified selected or deselected records.
      // This function is only used when the cardinality is greater than 1.
      function updateMany(filterOne, added, removed) {
        if (filterOne === one || resetNeeded) return;

        var i,
            k,
            n,
            g;

        // Add the added values.
        for (i = 0, n = added.length; i < n; ++i) {
          if (!(filters[k = added[i]] & zero)) {
            g = groups[groupIndex[k]];
            g.value = reduceAdd(g.value, data[k]);
          }
        }

        // Remove the removed values.
        for (i = 0, n = removed.length; i < n; ++i) {
          if ((filters[k = removed[i]] & zero) === filterOne) {
            g = groups[groupIndex[k]];
            g.value = reduceRemove(g.value, data[k]);
          }
        }
      }

      // Reduces the specified selected or deselected records.
      // This function is only used when the cardinality is 1.
      function updateOne(filterOne, added, removed) {
        if (filterOne === one || resetNeeded) return;

        var i,
            k,
            n,
            g = groups[0];

        // Add the added values.
        for (i = 0, n = added.length; i < n; ++i) {
          if (!(filters[k = added[i]] & zero)) {
            g.value = reduceAdd(g.value, data[k]);
          }
        }

        // Remove the removed values.
        for (i = 0, n = removed.length; i < n; ++i) {
          if ((filters[k = removed[i]] & zero) === filterOne) {
            g.value = reduceRemove(g.value, data[k]);
          }
        }
      }

      // Recomputes the group reduce values from scratch.
      // This function is only used when the cardinality is greater than 1.
      function resetMany() {
        var i,
            g;

        // Reset all group values.
        for (i = 0; i < k; ++i) {
          groups[i].value = reduceInitial();
        }

        // Add any selected records.
        for (i = 0; i < n; ++i) {
          if (!(filters[i] & zero)) {
            g = groups[groupIndex[i]];
            g.value = reduceAdd(g.value, data[i]);
          }
        }
      }

      // Recomputes the group reduce values from scratch.
      // This function is only used when the cardinality is 1.
      function resetOne() {
        var i,
            g = groups[0];

        // Reset the singleton group values.
        g.value = reduceInitial();

        // Add any selected records.
        for (i = 0; i < n; ++i) {
          if (!(filters[i] & zero)) {
            g.value = reduceAdd(g.value, data[i]);
          }
        }
      }

      // Returns the array of group values, in the dimension's natural order.
      function all() {
        if (resetNeeded) reset(), resetNeeded = false;
        return groups;
      }

      // Returns a new array containing the top K group values, in reduce order.
      function top(k) {
        var top = select(all(), 0, groups.length, k);
        return heap.sort(top, 0, top.length);
      }

      // Sets the reduce behavior for this group to use the specified functions.
      // This method lazily recomputes the reduce values, waiting until needed.
      function reduce(add, remove, initial) {
        reduceAdd = add;
        reduceRemove = remove;
        reduceInitial = initial;
        resetNeeded = true;
        return group;
      }

      // A convenience method for reducing by count.
      function reduceCount() {
        return reduce(crossfilter_reduceIncrement, crossfilter_reduceDecrement, crossfilter_zero);
      }

      // A convenience method for reducing by sum(value).
      function reduceSum(value) {
        return reduce(crossfilter_reduceAdd(value), crossfilter_reduceSubtract(value), crossfilter_zero);
      }

      // Sets the reduce order, using the specified accessor.
      function order(value) {
        select = heapselect_by(valueOf);
        heap = heap_by(valueOf);
        function valueOf(d) { return value(d.value); }
        return group;
      }

      // A convenience method for natural ordering by reduce value.
      function orderNatural() {
        return order(crossfilter_identity);
      }

      // Returns the cardinality of this group, irrespective of any filters.
      function size() {
        return k;
      }

      // Removes this group and associated event listeners.
      function dispose() {
        var i = filterListeners.indexOf(update);
        if (i >= 0) filterListeners.splice(i, 1);
        i = indexListeners.indexOf(add);
        if (i >= 0) indexListeners.splice(i, 1);
        i = removeDataListeners.indexOf(removeData);
        if (i >= 0) removeDataListeners.splice(i, 1);
        return group;
      }

      return reduceCount().orderNatural();
    }

    // A convenience function for generating a singleton group.
    function groupAll() {
      var g = group(crossfilter_null), all = g.all;
      delete g.all;
      delete g.top;
      delete g.order;
      delete g.orderNatural;
      delete g.size;
      g.value = function() { return all()[0].value; };
      return g;
    }

    // Removes this dimension and associated groups and event listeners.
    function dispose() {
      dimensionGroups.forEach(function(group) { group.dispose(); });
      var i = dataListeners.indexOf(preAdd);
      if (i >= 0) dataListeners.splice(i, 1);
      i = dataListeners.indexOf(postAdd);
      if (i >= 0) dataListeners.splice(i, 1);
      i = removeDataListeners.indexOf(removeData);
      if (i >= 0) removeDataListeners.splice(i, 1);
      m &= zero;
      return filterAll();
    }

    return dimension;
  }

  // A convenience method for groupAll on a dummy dimension.
  // This implementation can be optimized since it always has cardinality 1.
  function groupAll() {
    var group = {
      reduce: reduce,
      reduceCount: reduceCount,
      reduceSum: reduceSum,
      value: value,
      dispose: dispose,
      remove: dispose // for backwards-compatibility
    };

    var reduceValue,
        reduceAdd,
        reduceRemove,
        reduceInitial,
        resetNeeded = true;

    // The group listens to the crossfilter for when any dimension changes, so
    // that it can update the reduce value. It must also listen to the parent
    // dimension for when data is added.
    filterListeners.push(update);
    dataListeners.push(add);

    // For consistency; actually a no-op since resetNeeded is true.
    add(data, 0, n);

    // Incorporates the specified new values into this group.
    function add(newData, n0) {
      var i;

      if (resetNeeded) return;

      // Add the added values.
      for (i = n0; i < n; ++i) {
        if (!filters[i]) {
          reduceValue = reduceAdd(reduceValue, data[i]);
        }
      }
    }

    // Reduces the specified selected or deselected records.
    function update(filterOne, added, removed) {
      var i,
          k,
          n;

      if (resetNeeded) return;

      // Add the added values.
      for (i = 0, n = added.length; i < n; ++i) {
        if (!filters[k = added[i]]) {
          reduceValue = reduceAdd(reduceValue, data[k]);
        }
      }

      // Remove the removed values.
      for (i = 0, n = removed.length; i < n; ++i) {
        if (filters[k = removed[i]] === filterOne) {
          reduceValue = reduceRemove(reduceValue, data[k]);
        }
      }
    }

    // Recomputes the group reduce value from scratch.
    function reset() {
      var i;

      reduceValue = reduceInitial();

      for (i = 0; i < n; ++i) {
        if (!filters[i]) {
          reduceValue = reduceAdd(reduceValue, data[i]);
        }
      }
    }

    // Sets the reduce behavior for this group to use the specified functions.
    // This method lazily recomputes the reduce value, waiting until needed.
    function reduce(add, remove, initial) {
      reduceAdd = add;
      reduceRemove = remove;
      reduceInitial = initial;
      resetNeeded = true;
      return group;
    }

    // A convenience method for reducing by count.
    function reduceCount() {
      return reduce(crossfilter_reduceIncrement, crossfilter_reduceDecrement, crossfilter_zero);
    }

    // A convenience method for reducing by sum(value).
    function reduceSum(value) {
      return reduce(crossfilter_reduceAdd(value), crossfilter_reduceSubtract(value), crossfilter_zero);
    }

    // Returns the computed reduce value.
    function value() {
      if (resetNeeded) reset(), resetNeeded = false;
      return reduceValue;
    }

    // Removes this group and associated event listeners.
    function dispose() {
      var i = filterListeners.indexOf(update);
      if (i >= 0) filterListeners.splice(i);
      i = dataListeners.indexOf(add);
      if (i >= 0) dataListeners.splice(i);
      return group;
    }

    return reduceCount();
  }

  // Returns the number of records in this crossfilter, irrespective of any filters.
  function size() {
    return n;
  }

  return arguments.length
      ? add(arguments[0])
      : crossfilter;
}

// Returns an array of size n, big enough to store ids up to m.
function crossfilter_index(n, m) {
  return (m < 0x101
      ? crossfilter_array8 : m < 0x10001
      ? crossfilter_array16
      : crossfilter_array32)(n);
}

// Constructs a new array of size n, with sequential values from 0 to n - 1.
function crossfilter_range(n) {
  var range = crossfilter_index(n, n);
  for (var i = -1; ++i < n;) range[i] = i;
  return range;
}

function crossfilter_capacity(w) {
  return w === 8
      ? 0x100 : w === 16
      ? 0x10000
      : 0x100000000;
}